1. Field of the Invention
The present invention relates generally to nuclear fuel assemblies for a nuclear reactor and, more particularly, is concerned with a fuel assembly top nozzle incorporating an improved peripheral hold-down assembly.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle.
In the conventional fuel assembly, coolant flowing upward past the fuel rods and guide thimbles induces significant upward forces. These forces are countered by a combination of the weight of the fuel assembly and a resiliently yieldable hold-down assembly on the top nozzle which pushes against the upper core plate of the reactor. The hold-down assembly thereby provides a downward force which counteracts and prevents the force of the upward coolant flow from lifting the fuel assembly into damaging contact with the upper core plate, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like.
One prior art hold-down assembly employs a single large diameter helical coil spring centrally in the top nozzle to hold down the fuel assembly. The single coil spring used in this one prior art hold-down assembly has a tendency to fracture and fail due to vibration induced by coolant flow through the fuel assembly. Also, because of its susceptibility to failure, the single coil spring cannot withstand the large loads imposed on it when the flow of coolant increases due to activation of a final, or fourth, reactor coolant pump. As a result, the nuclear power plant operator must refrain from activating the last pump until the coolant reaches its start-up temperature of 450.degree.-550.degree. F., when the density of the coolant is less than at a lower temperature.
An improved hold-down assembly requiring only minimal modification of the overall top nozzle structure is proposed in the U. S. patent application cross-referenced above. In this improved hold-down assembly, the central coil spring is advantageously replaced by a diagonal arrangement of leaf springs which can withstand greater strain than the single coil spring and are not susceptible to failure caused by flow-induced vibration. The plant operator can now activate the fourth coolant pump for operation at higher coolant density and thus at lower coolant temperatures, for example, 300.degree. F.
However, one drawback of this improved hold-down assembly is that the diagonal springs and upper hold-down or retainer plate are positioned across the interior of the top nozzle where they obstruct coolant flow upwardly through holes defined in the adapter plate of the top nozzle. This arrangement results in a decrease in flow rate and increase in pressure drop of coolant flow through the top nozzle.
Consequently, a need still exists for an alternative design of a hold-down assembly to overcome the remaining problems described above without requiring a significant modification of the overall top nozzle structure.